What the is the difference between a locked and unlocked bootloader?

[callofduty4]

Im confused what it means when HTC says they have unlocked them. What does this enable us to do

 

[DenverRalphy]

The Bootloader is the first set of instructions executed at the device's startup. In simple terms, it gives the device instructions on where to find the operating system to be booted. If the bootloader isn't locked, it can be edited to provide alternate instructions, giving you the ability to boot additional or custom kernels/ROMS.

Just like on a PC where you can have more than one operating system installed. For example, the PC I'm currently typing on can be booted into either Windows or Linux.

 

[Jalarm]

If its unlocked its easier to root and use custom roms

 

[Jerry Hildenbrand]

http://www.androidcentral.com/bootloaders-all-you-ever-wanted-know

I try to explain it a bit here, without all the technical jargon.